1. Field of the Invention
The present invention relates to a novel discharge lamp lighting circuit equipped with a control function to supply power exceeding the rated power to a discharge lamp to thereby propagate light.
2. Description of the Related Art
There is a known lighting circuit for a discharge lamp like a metal halide lamp, which supplies more power than the rated one to a discharge lamp in a transient state from the beginning of the lighting of the discharge lamp to the transition of constant power control in order to shorten the ignition time, thereby expediting lighting.
In the case where, with the service life of a discharge lamp at its last stage, the lamp voltage does not fall within, or comes off, the proper voltage range under constant power control, the conventional lighting circuit supplies power exceeding the rated power to the discharge lamp. If this state continues for a long period of time, heat generation from the circuit or some other overpower induced shortcoming occurs, and the circuit may be damaged at the worst.
One solution to the problem is to check if the lamp voltage or the lamp current of a discharge lamp deviates from a predetermined range and stop power supply to the discharge lamp when such matter happens.
This scheme disables one who has intended to light the discharge lamp from knowing the reason for turning off the lamp, or requires some means to inform the person of that reason.